12 December 2008
If the full moon tonight looks unusually large, it is not your imagination – it is the biggest and brightest full moon to be seen for 15 years.
Each month the Moon makes a full orbit around the Earth in a slightly oval-shaped path, and tonight it will swing by the Earth at its closest distance, or perigee. It will pass by 356,613km (221,595 miles) away, which is about 28,000km closer than average.
The unusual feature of tonight is that the perigee also coincides with a full moon, which will make it appear 14 per cent bigger and some 30 per cent brighter than most full moons this year – so long as the clouds hold off from blocking the view.
The next closest encounter with a full moon this large will not be until November 14, 2016.
In addition to this lunar flypast, we may also be treated to a phenomenon known as the moon illusion. As the Moon rises in the late afternoon, it will appear even larger as it lies close to the horizon. Psychologists have tried to explain this as a trick of the eye, as the landscape on the horizon appears to make the Moon loom much larger, an effect that disappears as the Moon rises above the horizon, although viewing it through a tube, such as a toilet roll, can make it look large again.
With the Moon approaching so close to the Earth, its gravity will pull a slightly higher tide than normal for a full moon. This tide adds about 0.5m (1.6ft) to the high-water mark, and with freshening southwesterly winds forecast, this may cause some flooding, especially along parts of the South West coast.
Tonight’s full moon is also notable for rising to its greatest height in the night sky for the entire year, lying almost overhead at midnight. This is because we are approaching the winter solstice, on December 21, and thanks to the tilt of the Earth the Moon appears at its highest, as the Sun is at its lowest.
Another astronomical treat that could be seen tonight and for the next two nights is the annual Geminid meteor shower, one of the year’s best displays of shooting stars. Up to 100 meteors an hour can fly across the sky. The meteors, which are easy to spot with the naked eye, appear to shoot out from the constellation Gemini, hence their name, but they can be seen all over the sky. However, with a full moon so bright, the best place to look is away from the Moon.
Meteor showers happen when the Earth passes through clouds of debris shed from comets. As the tiny fragments smash into the Earth’s upper atmosphere at about 100,000mph, they burn up in streaks of light.
For reasons that are not understood, the Geminid meteor showers are tending to grow stronger each year.